The present invention relates generally to an exposure apparatus, and more particularly to an exposure apparatus used to manufacture various devices including semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, sensing devices such as magnetic heads, and image pickup devices such as CCDs, as well as fine patterns used for micromechanics. The present invention is suitable, for example, for a step-and-scan exposure apparatus in the lithography process.
Conventionally employed reduction projection exposure apparatuses use a projection optical system to project or transfer a circuit pattern on a mask or a reticle onto a wafer, etc., in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology.
Recently, as shown in FIG. 13, in order to improve the resolution and enlarge the exposure apparatus, a scanning projection exposure apparatus (also referred to as a “scanner”) has been conventionally used to expose part of the reticle RT continuously and expose the entire reticle pattern onto each exposure area on the wafer WF by synchronously scanning the reticle RT and the wafer WF relative to a projection optical system 1200 by using a reticle stage 1100 and a wafer stage 1300. FIG. 13 is a schematic perspective view showing a structure of the conventional scanning exposure apparatus 1000.
In order to align the reticle with the wafer during the exposure, the exposure apparatus includes plural alignment optical systems. For example, the alignment optical system is includes, as shown in FIG. 13, an off-axis alignment optical system 1600 that detects an alignment mark on the wafer WF for the alignments of the wafer, and a through the reticle (“TTR”) alignment optical system 1500 that detects, via the projection optical system 1200, an alignment mark on the wafer WF corresponding to the alignment mark on the reticle RT. The TTR alignment optical system is also referred to as a through the lens (“TTL”) alignment optical system.
A focus/leveling detecting system 1400 of an oblique incident system is configured as a wafer surface detecting means. The focus/leveling detecting system 1400 irradiates, via the projection optical system 1200, the light oblique to the wafer surface on which a pattern on the reticle RT is transferred (or a reference plate 1700 surface), and detects the reflected light reflected obliquely from the wafer surface or the reference plate 1700).
The focus detecting system 1400 includes as a detector a position detecting, light receiving element corresponding to the reflected light so that the light receiving surface of the position detecting light receiving element and the reflecting point of the light on the wafer WF are approximately conjugate to each other.
Therefore, a positional offset in the optical-axis direction of the projection optical system 1200 of the wafer WF (or the reference plate 1700) is measured as a positional offset on the position detecting light receiving element in the detector.
In particular, plural position detecting light receiving elements corresponding to plural rays are provided so as to detect leveling (or a tilt) as well as the focus. A light receiving surface of each position detecting light receiving element and the reflecting surface of each ray on the wafer WF are made approximately conjugate to each other. A tilt of the wafer WF (or a reference plate 1700) is measured from the focus measuring results at plural measuring points.
Te TTL optical system 1500 also serves as a focus calibration function for measuring errors caused by the measuring origin of the focus/leveling detecting system 1400 and the focal surface of the projection optical system 1200 when the projection optical system 1200 absorbs the exposure heat or the surrounding environment changes. The TTL optical system 1500 generally includes two optical systems 1500a and 1500b, and can measure focus states of the projection optical system 1200 at two points simultaneously.
The scanning exposure apparatus defines a rectangular or arc-shaped slit exposure area at the still time where a long side aligns with a direction orthogonal to a scan direction and a short side aligns with the scan direction, and deteriorates the resolution when the image surface of the projection optical system and the actual exposure surface or the wafer surface incline to each other in the long side direction.
Accordingly, the TTL optical system measures the focus state of the projection optical system at two points in the exposure slit so as to calculate the tilt of the image surface in the long side direction. Then, the good resolution performance is obtained by according the actual exposure surface with this tilt (see, for example, Japanese Patent Application No. 5-45889).
The recent high integration of the semiconductor devices demands the finer processing to patterns to be transferred or high resolution. The prior art has attempted to meet this requirement by using the exposure light having a short wavelength. However, only the short wavelength of the exposure light cannot satisfy the rapidly progressing integration of the semiconductor devices. For the high resolution, along with the short wavelength, the high numerical aperture (“NA”) of the projection optical system has recently shifted from about 0.6 to 0.8 or higher.
This configuration, however, makes a depth of focus (“DOF”) excessively small, and demands for the remarkably improved detecting accuracy of the focal point, in particular, improved accuracy of the focus calibration. As the DOF decreases, it is vital to measure the tilt of the image surface in the scan direction, which has been conventionally negligible, for example, to accord the actual exposure surface with the image surface of the projection optical system by driving the wafer stage, and to correctively accord the image surface side with the actual exposure surface by driving the lens, etc. in the projection optical system. It is possible to measure and correct the tilt of the image surface in the long side direction, as described above, in the conventional scanning exposure apparatus, but it is not possible to measure or correct the tile of the image surface in the scan direction.
When the focus/leveling detecting system's reference surface varies with time, the best exposure deteriorates. In order to correct this fluctuation, the focus/leveling detecting system is corrected on the basis of a so-called running surface by sequentially feeding the reference mark on the wafer stage to plural measuring points prior to the measurements. However, when the stage running surface does not accord with the tilt of the image surface of the projection optical system, the reference surface correction of the focus/leveling detecting system deteriorates.
The exposure apparatus is required to improve the throughput (or the number of sheets exposed per unit of time) for improved the productivity, and needs to shorten the measuring time while improving the measuring accuracy in the focus calibration.